The present invention relates to an epoxy resin composition and to a method for preparing electrical laminates using said composition.
Heretofore, laminates used in the preparation of electrical circuit boards have been conventionally produced by impregnating a supporting or reinforcing web with a resin composition by passing the web through a bath containing the resin composition (conventionally referred to as a "varnish") and subsequently passing the impregnated web through a heated chamber where any solvent is removed and the resin partially cured or advanced. One or more layers of the impregnated web (conventionally referred to as a "prepreg") are then sandwiched between thin layers of an electrical conductive material such as copper foil under heat and pressure. The resulting construction, which is commonly referred to as an electrical laminate, is further processed into a circuit board.
An epoxy type resin is often employed for impregnating the reinforcing web. A typical epoxy resin varnish composition for such application comprises a brominated epoxy resin prepared from a diglycidyl ether of bisphenol A and a tetrabromobisphenol A, dicyanidiamide as a hardener or curing agent, an organic solvent such as acetone and an accelerator. Unfortunately, due to relative instability of the composition, the epoxy resin, hardener, solvent and accelerator or catalyst must be combined shortly prior to the intended use of the composition. In addition, the dicyandiamide curing agent is often of very limited solubility in organic liquid and therefore tends to crystallize in the varnish composition and/or on the prepeg made from the varnish. To maintain the desired solubility requires that the solvent be selected from a relatively narrow group of compounds and large amounts of solvent be employed.
An alternative varnish composition for applying to the reinforcing web used in the preparation of electrical laminates comprises a blend of a diglycidyl ether of bisphenol A and either bisphenol A or tetrabromobisphenol A, a dicyandiamide curing agent, a solvent and, optionally, an accelerator. The diglycidyl ether of bisphenol A is reacted in situ with the bisphenol A and/or tetrabromobisphenol A and dicyandiamide during impregnation of the reinforcing web and advancement of the resin. See, for example, U.S. Pat. No. 3,738,862. Unfortunately, the epoxy resin, polyhydric phenol, solvent, curing agent and accelerator must again be combined shortly prior to use to prevent increases in viscosity or crystallization of the hardener from the composition.
Yet another composition has been proposed for use in the preparation of electrical laminates. This composition comprises an epoxy resin, an organic solvent, a hardener derived from a hexa-alkyl ether of a hexamethylol melamine and, optionally, a polyhydric phenol and an accelerator. Unfortunately, although not as severe as with the other known varnish compositions, the described varnish composition still exhibits some increases in viscosity over time.
In all cases, similar composition to those described except containing no accelerator will exhibit increased stability and, hence a longer shelf life. Specifically, the composition is prepared without the accelerator and the accelerator is added only shortly prior to the actual use of the varnish composition in the laminating/coating operation. However, the shelf life of the composition is still limited due to the large increases in viscosity experienced by the composition over a relatively short period of time. For example, although the shelf life is dependent on the solids content and the specific solvent employed, a composition comprising a liquid epoxy resin such as the diglycidyl ether of bisphenol A, a polyhydric alcohol such as bisphenol A and dicyanamide will often possess a shelf life of only 2-3 weeks prior to the viscosity of the composition increasing sufficiently to render the composition unusable or the dicyanamide coming out of solution.
In view of the deficiencies of the prior art varnish compositions for use in the preparation of electrical laminates, it would be highly desirable to provide a varnish composition based on a stable epoxy resin/hardener system which does not possess the deficiencies of the prior art.